This invention relates generally to fiber optic detection systems based on the Sagnac interferometer. In one mode of operation the Sagnac interferometer disclosed by this invention responds to time varying environmental disturbances in a manner similar to that described by Richard Cahill and Eric Udd in U.S. Pat. No. 4,375,680, "Optical Acoustic Sensor" and by Eric Udd in "Fiber-Optic Acoustic Sensor Based on the Sagnac Interferometer", Proceedings of SPIE, Vol. 425, p. 90, 1983. There it was shown that the amplitude of the response to a time varying disturbance depends upon its position in the Sagnac loop where the sensing occurs. If the disturbance occurs near the center of the loop, the response becomes vanishingly small, while for a disturbance near the ends of the loop, the response approaches the maximum possible amplitude. In the second mode of operation, the Sagnac interferometer is configured to have sensitivity that is constant over the length of the sensing loop. These configurations are described in U.S. patent application Ser. No. 917,390, which issued on Nov. 29, 1988 as U.S. Pat. No. 4,787,741, applied for by Eric Udd, et al, and assigned to the assignee of the instant application.
John Dakin, in a paper entitled "A Novel Distributed Optical Fiber Sensing System Enabling Location of a Disturbance in a Sagnac Loop Interferometer", Proceedings of the SPIE, v. 838, p. 325 (1987), describes a combination of Mach-Zehnder and Sagnac interferometers where along a single fiber optic path the Mach-Zender interferometer has direct detection sensitivity while the Sagnac interferometer has position dependent sensitivity as noted above. By ratioing the position dependent and position independent signals, the location and magnitude of the disturbance may be determined. This latter invention is severely limited by the contradictory requirements of the Mach-Zehnder and Sagnac interferometers. For the Mach-Zehnder interferometer optimum performance is achieved by utilizing a long coherence length light source with high frequency stability. The performance of these light sources degrade rapidly with light feedback into the source. The Sagnac interferometer has optimum performance when a low coherence length light source is used, and its performance degrades rapidly as the coherence length increases due to Rayleigh backscatter from the sensing loop. The contradictory requirements of these interferometers result in a light source which is a compromise resulting in substantial limitations in the performance of one or both interferometers in the Dakin device severely limiting performance. The situation is further aggravated by the combination of Sagnac and Mach-Zehnder interferometer as described by Dakin resulting in direct feedback of the signal light into the light source, which results in a worst case scenario for the light source that would optimize the performance of the Mach-Zehnder.
What is desired is a fiber optic sensor that is capable of sensing both the location and magnitude of a disturbance along a single fiber without the limitations and excess noise generated by mixing the highly incompatible Sagnac and Mach-Zehnder interferometers.